Project summary The overall goal of the proposed study is to understand the mechanism underlying allosteric regulation of ADAMTS13 function, to determine the therapeutic efficacy of rADAMTS13-loaded platelets in acquired thrombotic thrombocytopenic purpura (TTP), and to elucidate the mechanism underlying platelet uptake of rADAMTS13. In Aim 1, we will use the novel deuterium-hydrogen (HX) and mass spectrometry (MS) and other biophysical tools to determine the role of distal C-terminal domains in allosteric and pH-dependent regulation of ADAMTS13 function; In Aim 2, we will determine therapeutic efficacy of ADAMTS13-loaded platelets in murine and human models of acquired TTP; and In Aim 3, we will determine the mechanism underlying platelet endocytosis of ADAMTS13 by using ?3-/-, arf6-/-, and VAMP3-/- mice. We believe that the results of the proposed study will shed new light on the fundamental biology of ADAMTS13, help to understand the pathogenesis of acquired TTP, and to develop a novel therapeutic strategy for this fatal syndrome and perhaps other arterial thrombotic disorders.